Excerpts from Books and Wikipedia
"The Irish Civil War (...) was a conflict that followed the Irish War of Independence and accompanied the establishment of the Irish Free State, an entity independent from the United Kingdom but within the British Empire. The civil war was waged between two opposing groups, Irish republicans and Irish nationalists, over the Anglo-Irish Treaty. The forces of the Provisional Government (which became the Free State in December 1922) supported the Treaty, while the Republican opposition saw it as a betrayal of the Irish Republic (which had been proclaimed during the Easter Rising). Many of those who fought on both sides in the conflict had been members of the Irish Republican Army (IRA) during the War of Independence. The Civil War was won by the Free State forces, who benefitted from substantial quantities of weapons provided by the British Government. The conflict may have claimed more lives than the War of Independence that preceded it, and left Irish society divided and embittered for generations. Today, two of the main political parties in the Republic of Ireland, Fianna Fáil and Fine Gael, are direct descendants of the opposing sides of the war." [Wikipedia]